Remove pbr warnerrors
This change removes the now unused "warnerrors" setting,
which is replaced by "warning-is-error" in sphinx
releases >= 1.5 [1].
[1] http://lists.openstack.org/pipermail/openstack-dev/2017-March/113085.html
Change-Id: I32f078169668be08737e47cd15edbdfba42904dc
(cherry picked from commit f76a628d1f)
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